Exhaust gas turbocharger

ABSTRACT

The invention concerns an exhaust gas turbocharger ( 1 ) having a shaft ( 2 ) on which a turbine wheel ( 4 ) and a compressor wheel ( 3 ) sit and which is guided in radial bearings ( 5, 6 ) and in at least one axial bearing ( 9 ), which is characterized in that the radial bearings ( 5, 6 ) are embodied as passive magnetic bearings having permanent magnets ( 23  through  31 ) generating axial magnetic fluxes; and the axial bearing ( 9 ) is embodied as an active magnetic bearing having an electromagnet ( 55 ), an axial sensor, and a controller for controlling the electrical current impinging upon the electromagnet ( 55 ).

The invention concerns an exhaust gas turbocharger having a shaft onwhich a turbine wheel and a compressor wheel sit and which is guided inradial bearings and in at least one axial bearing.

Exhaust gas turbochargers serve to improve the efficiency and thusincrease the output of internal combustion engines. They comprise ashaft that is equipped at one end with a turbine wheel and at the otherend with a compressor wheel. The turbine wheel is impinged upon by theexhaust gas flow of the internal combustion engine, the kinetic energyof the exhaust gas being substantially converted by the turbine wheelinto a rotary motion. The shaft drives the compressor wheel, which drawsin fresh air and causes it to flow with positive pressure into theintake ducts of the internal combustion engine, thus improvingvolumetric efficiency.

Severe demands are imposed on the bearing system for exhaust gasturbocharger shafts. On the one hand, the shaft reaches high rotationspeeds of up to 300,000 rpm. On the other hand, the exhaust gasturbochargers and thus their bearings are exposed to high temperatures.A further problem is that the exhaust gas flow striking the turbinewheel generates strong axial forces that must be absorbed in an axialbearing. Because of the high rotation speeds, the rotating parts of theexhaust gas turbocharger must be very accurately balanced so that as fewvibrations as possible are generated. With all of this, care mustadditionally be taken that the very wide temperature range in which anexhaust gas turbocharger operates does not result in distortions of thebearings as a result of material expansion.

The bearings hitherto used for the shaft have hitherto been exclusivelyplain or rolling bearings. In view of the aforementioned stresses, theyare subject to considerable wear and, along with their lubrication, areresponsible for approximately 80% of the failures in an exhaust gasturbocharger.

It is the object of the invention to configure the bearing system of anexhaust gas turbocharger in such a way that it has as little wear aspossible, is less subject to malfunction, and has no tendency tovibrate.

According to the present invention, this object is achieved in that theradial bearings are embodied as passive magnetic bearings havingpermanent magnets generating axial magnetic fluxes; and the axialbearing is embodied as an active magnetic bearing having anelectromagnet, an axial sensor, and a controller for controlling theelectrical current impinging upon the electromagnet. The basic idea ofthe invention is thus to support the exhaust gas turbocharger shaft inentirely magnetic fashion, by the fact that the shaft is held infloating fashion in the magnetic bearings and no mechanical contactexists with stationary parts, and there is therefore also no mechanicalfriction. The magnetic bearing system for the exhaust gas turbochargershaft does not constitute an upper limit in terms of rotation speed. Itis moreover characterized by better efficiency in the absence ofmechanical friction. A magnetic bearing system is also maintenance-freeand requires no lubrication.

Magnetic bearings have been proposed in many embodiments in other fieldsof technology, for example in vacuum pumps, blood pumps, gyroscopicdevices, or in spinning rotors. Purely by way of example, the reader isreferred to the following documents out of the many publicationsdisclosed in this context: U.S. Pat. Nos. 3,976,339, 5,315,197,5,514,924, 4,620,752, WO 92/15795, U.S. Pat. No. 5,729,065, WO 00/64030,WO 00/64031. Although such bearing systems have already been known forsome time, and exhaust gas turbochargers have likewise been in commonuse for decades, because of the stringent requirements it has apparentlynot been conceivable that the shaft of an exhaust gas turbocharger canbe supported in entirely magnetic fashion.

In the present case this is done by the fact that at least two passivemagnetic bearings are combined with one active magnetic bearing. Morethan two passive and more than one active magnetic bearing can also bepresent, and the active and passive magnetic bearing can form a unit.The passive magnetic bearings have permanent magnets in the stationarypart and usefully also in the rotating part, said magnets being arrangedso that in the gaps between the rotating and stationary parts, an axialattractive magnetic flux is generated that opposes any radialdisplacement of the shaft. By way of the strength, number, andarrangement of the permanent magnets it is thus possible to generate amagnetic flux which is sufficiently strong that the shaft is held infloating fashion even when strong external forces are applied. Balancingof the shaft is no longer necessary, or at least need not be aslaborious as with previously known exhaust gas turbochargers. It isworth considering in this context that the balancing process can accountfor up to 15% of the manufacturing cost of the exhaust gas turbocharger.

The axial instability of the shaft is compensated for by theelectromagnetic axial bearing, which is part of a control loop having acontroller that comprises a sensor for sensing the axial motion of theshaft. Active electromagnetic axial bearings of this kind are known perse in the existing art (cf. U.S. Pat. Nos. 5,315,197, 4,620,752, WO92/15795, U.S. Pat. No. 5,729,065). The controller adjusts the currentacting on the coil, in terms of polarity and strength, in such a waythat the coil generates a magnetic field that acts axially on the shaftand is opposite to the respective deflection of the shaft in the axialdirection, and thus moves the shaft back into the defined position. Thishappens so quickly that the shaft executes practically no axial motions.

It has been found in this context that with an active magnetic bearingof this kind it is possible to absorb even the large axial forces thatoccur in exhaust gas turbochargers. The possibility also exists ofaxially preloading the axial bearing, using additional permanentmagnets, in such a way that the average axial force acting on the shaftduring operation is absorbed by these permanent magnets, and theelectromagnet is impinged upon by electrical current only in order tocompensate for axial forces greater than or less than the average value.The electrical current to be expended for axial stabilization canthereby be kept low.

In an embodiment of the invention, provision is made for two radialbearings to be present, between which at least one axial bearing isarranged. The radial bearings within the turbocharger should have thegreatest possible spacing so as to yield a large lever arm for anytilting motions of the shaft that may occur.

In a particularly preferred embodiment, it is proposed that the axialbearing have a radially projecting bearing ring made of magnetizablematerial and at least one yoke, made of ferromagnetic material andforming an axial bearing stator, that encloses the bearing ring on bothsides of the bearing ring forming magnetic gaps; and that at least onepair of axially oppositely polarized permanent magnets be arrangedaxially next to one another in the yoke, and an electromagnetic coilalso be arranged radially adjacently as the electromagnet, the magneticflux in the coil and thus in the magnetic gaps being controllable by wayof the controller in such a way that the bearing ring is held in theyoke axially in a defined position.

The basic idea of this particular embodiment is that the axial bearingcomprises a combination of a yoke, a coil, and two oppositely polarizedpermanent magnets arranged axially next to one another, the coil andpermanent magnets lying radially next to one another. Four partialmagnetic fluxes are thus constituted in the yoke, of which two arelocated axially and two radially next to one another. Two of the partialmagnetic fluxes penetrate the bearing ring connected to the shaft, andgenerate axially and oppositely directed magnetic fields in the magneticgaps. The other two partial magnetic fluxes penetrate into the yoke fromthe outside. The magnetic fields in the magnetic gaps on the bearingring can be asymmetrically influenced by current impingement on thecoil, in such a way that the magnetic field is strengthened in the onemagnetic gap and weakened in the other. An axial force is thus exertedon the bearing ring and thus on the shaft. This force counteracts anyaxial offset of the shaft, the sensor of the controller sensing thisaxial motion and controlling current delivery to the coil in such a waythat the bearing ring and thus the shaft is axially centered in theyoke.

Although, as shown by the documents cited above, active axial bearingsmade up of a combination of electromagnetic coils and permanent magnetsare already known, the embodiment claimed here is neverthelessdistinguished from them by a simple physical configuration (only onecoil is necessary) and by a large transfer of force in the axialdirection, since only two magnetic gaps are present and theforce/current characteristic of the coil is not negatively affected bythe placement of the magnetically poorly conductive permanent magnets.The axial bearing according to the present invention is thusparticularly suitable for absorbing the large axial forces that act onthe shaft, and simultaneously holding the shaft in a defined position.It is understood in this context that multiple permanent magnets and/orcoils can also be provided radially next to one another.

The possibility exists of arranging several axial bearings of the kinddescribed above distributed over the circumference of the bearing ring.It is simpler in terms of design, however, to embody the axial bearingas an annular bearing having a yoke, configured as an annular yoke, thatsurrounds the bearing ring. The permanent magnets are advantageouslyembodied as axially magnetized annular magnets, and the coil as anannular coil.

To generate a magnetic flux with as little loss as possible, thepermanent magnets should be in contact without gaps against the yoke andagainst each other. For the same reason, the coil should be in contactwithout gaps against the yoke and against the permanent magnets. Onlythe magnetic gaps between the bearing ring and yoke therefore remain.

According to a further feature of the invention, provision is made forthe permanent magnets to be radially adjacent to the circumferentialside of the radial web, and for the coil to sit on the radially outwardside thereof. This results in a particularly favorable magnetic flux.

The radial bearings should each have a bearing ring sitting on theshaft, and a radial bearing stator located axially opposite that ring onat least one side, permanent magnets being provided both in the bearingring and in the radial bearing stator. Several permanent magnets shouldbe arranged next to one another in the radial direction, preferablybeing in contact with one another and being alternately oppositelypolarized; i.e. each two adjacent permanent magnets on the radialbearing stator or on the bearing ring are oppositely polarized.Particularly large magnetic forces are thereby generated.

It is possible in principle for the radial bearing stators to haveseveral partial stators, with permanent magnets, distributed over thecircumference. It is simpler in terms of design, however, to embody theradial bearing stators as annular stators and the permanent magnets asannular magnets.

In principle, it is sufficient for a radial bearing stator to beassociated with each bearing ring on only one side. The radial bearingstators can be arranged and embodied in such a way that the axial forcesdo not, as is normally the case, cancel one another out, but insteadthat an axial force, directed oppositely to the average axial forceacting on the shaft during operation, is continuously generated in onedirection. This can also be achieved with an embodiment in which thebearing ring is enclosed on both sides by radial bearing stators havingpermanent magnets. A particularly powerful magnetic flux, counteractingany radial deflection of the shaft, can thereby be achieved. It isunderstood that in this context, the radial bearings can also beconfigured differently, i.e. so that the bearing ring of the one radialbearing has a radial bearing stator on only one side, while the bearingring of the other radial bearing has radial bearing stators on bothsides. It is also understood that within one radial bearing, multiplebearing rings having a corresponding number of radial bearing statorscan also be provided. This case refers simply to a series arrangement ofmultiple radial bearings.

If two radial bearing stators are provided in one radial bearing, theyshould preferably be combined into a yoke that is U-shaped in crosssection.

Magnetic bearings have the property that they cause almost no damping.The present invention therefore provides that at least one radialbearing stator, preferably all the radial bearing stators, be supportedvia spring and damper elements in radially movable fashion on ahousing-mounted part of the exhaust gas turbocharger. This can be done,for example, by means of axially extending torsion springs; the radialbearing stator can be connected to the housing-mounted part via severaltorsion springs distributed over the circumference. The torsion springscan each be part of a cage that connects the ends of the torsion springsvia cage rings, and is coupled at one end to the radial bearing statorand at the other end to the housing-mounted part. To achieve aspace-saving configuration, the cage should surround the respectivelyassociated radial bearing stators.

It is additionally useful that the radial bearing stator suspended onspring elements is braced against the housing-mounted part by way of atleast one damping element that damps the radial excursions of the springelements. Each damping element can be embodied annularly and coaxiallywith respect to the shaft, and can be loaded either in compression or inshear. In a particular configuration, the damping element is embodied asa liquid film, preferably equipped with magnetic or magnetizableparticles, the liquid film being magnetically impinged upon on at leastone side by a permanent magnet that can be part of the passive magneticbearing. The liquid film is thus magnetically trapped. The viscosity ofthe liquid film can be adapted to the respective damping requirements.

The invention is illustrated in more detail in the drawings, withreference to an exemplary embodiment. In the drawings:

FIG. 1 is a side view of an exhaust gas turbocharger without thehousing, with a partially sectioned depiction of the upper part of theshaft bearing system;

FIG. 2 is a cross section through a radial bearing of the exhaust gasturbocharger shown in FIG. 1;

FIG. 3 is a perspective depiction of a spring cage for the radialbearing shown in FIG. 2;

FIG. 4 is an enlarged depiction of the axial bearing of the exhaust gasturbocharger shown in FIG.1; and

FIG. 5 shows the axial bearing according to FIG. 4 with activeinfluencing of the magnetic flux.

Exhaust gas turbocharger 1 depicted in FIG. 1 comprises a shaft 2 onwhose left end sits a compressor wheel 3, and on whose right end sits aturbine wheel 4. Compressor wheel 3 is embodied, in a manner known perse, as a radial compressor.

Two radial bearings 5, 6 are located between compressor wheel 3 andturbine wheel 4. Radial bearings 5, 6 are adjacent to compressor wheel 3and to turbine wheel 4, respectively. Arranged between them are grooves7, 8 that serve to receive sealing rings, which constitute delimitingbearings having a typical clearance of approx. +/−0.15 mm. An axialbearing 9 is located between radial bearings 5, 6.

As is apparent from the upper part of FIG. 1, shaft 2 is surrounded by atotal of six rings that are axially braced against a shoulder 10 onshaft 2. A first shaft sleeve 11 having groove 7 is followed by abearing washer 12, a second shaft sleeve 13, a bearing washer 14, athird shaft sleeve 15, and a further bearing washer 16.

Bearing washers 12, 16 belong to radial bearings 5, 6. They are eachenclosed on both sides by a yoke 17, 18 that is U-shaped in crosssection and coaxially surrounds shaft 2, each yoke 17, 18 comprising apair of radial bearing stators 19, 20 and 21, 22 that form the limbs ofyokes 17, 18. Radial bearing stators 19, 20, 21, 22, and bearing washers12, 16, have permanent magnets 23, 24, 25, 26 and 27, 28, 29, 30, whichare located respectively opposite the two radial bearings 5, 6 in theaxial direction. They are polarized in such a way that they attract oneanother, resulting in an axially directed and attractive magnetic fieldin the gaps between bearing washers 12, 16 and radial bearing stators19, 20, 21, 22. The magnetic fields center shaft 2, a radial stiffnessof 160 kN/m being achieved.

Permanent magnets 23 through 30 each comprise nine annular magnets(labeled 31 by way of example) set coaxially in one another, as isevident from the enlarged depiction of radial bearing 6 in FIG. 2. Theannular magnets 31 of a permanent magnet 23 through 30 are in contactagainst on another in the radial direction. Two annular magnets 31adjacent in the radial direction are oppositely axially magnetized. Theaxially oppositely located annular magnets 31 of two adjacent permanentmagnets 23 through 30 are polarized in mutually attractive fashion, sothat an axial magnetic flux is produced.

Yokes 17, 18 are surrounded externally by spring cages 35, 36 (omittedin the lower part of FIG. 1) that are connected at the outer edge toyokes 17, 18 and at the inner edge to housing washers 37, 38 (omitted inthe lower part of FIG. 1) that in turn are secured to a housing 39.Spring cage 36 is depicted individually in FIG. 3. At the edges it hastwo cage rings 40, 41 that are connected via eight regularly distributedspring struts (labeled 42 by way of example) extending in the axialdirection. Spring struts 42 permit a mutual parallel displacement of thetwo cage rings 40, 41, in which context spring struts bend in the radialdirection. Yokes 17, 18 can thus deflect radially.

Located between yokes 17, 18 and housing washers 37, 38 are narrow gaps,in each of which a damping ring 43, 44 is provided (FIG. 1). Dampingrings 43, 44 are made of a highly viscous liquid film containingmagnetic particles. The liquid film is stressed in shear in the contextof a radial motion of yokes 17, 18 and thus acts in damping fashion. Itis trapped in yokes 17, 18 by annular magnets 45, 46.

Bearing washer 14 belongs to axial bearing 9. It is enclosed on bothsides by an annular yoke 47 made of sheet Si iron. Annular yoke 47 isenclosed and secured between the two housing washers 37, 38. It has anouter yoke shell 48 from which proceed two inwardly directed yoke limbs49, 50 that have an L-shaped cross section and enclose bearing washer 14with limb segments directed toward one another, two magnetic gaps 51, 52being created. Located inside annular yoke 47 adjacent to thecircumferential side of bearing washer 14 are two permanent magnets 53,54, lying axially next to one another, which are polarized axiallyoppositely (symbolized by the triangles). They are in contact againstone another and against yoke limbs 49, 50. They are surrounded by anelectromagnetic annular coil 55 that fills up the space betweenpermanent magnets 53, 54 and yoke shell 48 and yoke limbs 49, 50.

As is evident in particular from FIG. 4, the two permanent magnets 53,54 produce a total of four partial magnetic fluxes 56, 57, 58, 59, theadjacent partial magnetic fluxes 56, 57, 58, 59 being directedoppositely to one another in each case. The inner partial magneticfluxes 56, 57 create axially directed magnetic fluxes in magnetic gaps51, 52, so that the surfaces located opposite one another in magneticgaps 51, 52 are mutually attracted. The magnetic forces cancel out whenbearing washer 14 is in the center position. The outer partial magneticfluxes 58, 59 pass through yoke limbs 49, 50 into yoke shell 48, andfrom there via annular coil 55 back into annular magnets 45, 46.

Because of the magnetic instability of shaft 2 in the axial direction,an axial stabilization must be effected by way of axial bearing 9. Thistakes place, in the context of an axial excursion of bearing washer 14,by the fact that this excursion is sensed by a sensor (not depicted hereand known in the existing art), and as a result the controller (also notdepicted) controls the delivery of current to annular coil 55 in such away that an additional magnetic flux is generated, resulting globally inan asymmetrical magnetic flux distribution within axial bearing 9. Thisis apparent from FIG. 5. In this case a minimal excursion of bearingwasher to the right is present. As a result, annular coil 55 is impingedupon by an electrical current having a direction such that thediagonally opposite partial magnetic fluxes 56, 59 are strengthened(symbolized by the denser flux lines), and the other partial magneticfluxes 57, 58 are weakened. As a result, the attractive force in leftmagnetic gap 51 increases, while the magnetic force in right magneticgap 52 weakens. The axial excursion of bearing washer 14 to the right istherefore opposed by a magnetic attractive force in the axial direction,with the result that bearing washer 14 becomes centered again withrespect to annular yoke 47.

1. An exhaust gas turbocharger comprising: a shaft; turbine wheel on theshaft; a compressor wheel on the shaft; passive magnetic radial bearingsguiding the shaft, the radial bearings comprising a bearing ring on theshaft, at least one radial bearing stator located axially opposite thebearing ring on at least one side, and permanent magnets provided bothon the bearing rings and on the radial bearing stator for generatingaxial magnetic fluxes; and at least one active magnetic axial bearingguiding the shaft, the axial bearing comprising an electromagnet, anaxial sensor, and a controller for controlling the electrical currentimpinging upon the electromagnet.
 2. An exhaust gas turbochargercomprising a shaft on which a turbine wheel and a compressor wheel sitand which is guided in radial bearings and in at least one axialbearing, wherein the radial bearings are embodied as passive magneticbearings having first permanent magnets generating axial magneticfluxes; and the axial bearing is embodied as an active magnetic bearinghaving an electromagnet, an axial sensor, and a controller forcontrolling the electrical current impinging upon the electromagnet, andwherein the axial bearing has a radially projecting bearing ring,sitting on the shaft and made of magnetizable material, and at least oneyoke, made of ferromagnetic material and forming at least one axialbearing stator, that encloses the bearing ring forming magnetic gaps;and at least one pair of axially oppositely polarized second permanentmagnets are arranged axially next to one another in the yoke, and anelectromagnetic coil is also arranged radially adjacently as theelectromagnet, the magnetic flux in the coil and thus in the magneticgaps being controllable by way of the controller in such a way that thebearing ring is held in the yoke axially in a defined position.
 3. Theexhaust gas turbocharger as defined in claim 2, wherein several axialbearings are provided, distributed over the circumference of the bearingring.
 4. The exhaust gas turbocharger as defined in claim 2, wherein thesecond permanent magnets are in contact against the yoke.
 5. The exhaustgas turbocharger as defined in claim 2, wherein the second permanentmagnets are in contact against each other.
 6. The exhaust gasturbocharger as defined in claim 2, wherein the coil is in contactagainst the yoke.
 7. The exhaust gas turbocharger as defined in claim 2,wherein the coil is in contact against the second permanent magnets. 8.The exhaust gas turbocharger as defined in claim 2, wherein the secondpermanent magnets are radially adjacent to the circumferential side ofthe radial ring, and the coil sits on the radially outward side thereof.9. The exhaust gas turbocharger as defined in claim 2, wherein two ofthe radial bearings are present, between which the at least one axialbearing is arranged.
 10. The exhaust gas turbocharger as defined inclaim 2, wherein the axial bearing is embodied as an annular bearing.11. The exhaust gas turbocharger as defined in claim 10, wherein theyoke is configured as an annular yoke that surrounds the bearing ring.12. The exhaust gas turbocharger as defined in claim 11, wherein thesecond permanent magnets are embodied as annular magnets, and the coilas an annular coil.
 13. An exhaust gas turbocharger comprising a shafton which a turbine wheel and a compressor wheel sit and which is guidedin radial bearings and in at least one axial bearing, wherein the radialbearings are embodied as passive magnetic bearings having permanentmagnets generating axial magnetic fluxes; and the axial bearing isembodied as an active magnetic bearing having an electromagnet, an axialsensor, and a controller for controlling the electrical currentimpinging upon the electromagnet, and wherein the radial bearings eachhave a bearing ring sitting on the shaft, and at least one radialbearing stator located axially opposite the bearing ring on at least oneside, the permanent magnets being provided both on the bearing rings andon the radial bearing stators.
 14. The exhaust gas turbocharger asdefined in claim 13, wherein the radial bearing stators have severalpartial stators, with permanent magnets, distributed over thecircumference.
 15. The exhaust gas turbocharger as defined in claim 13,wherein the radial bearing stators are embodied as annular stators andthe permanent magnets as annular magnets.
 16. The exhaust gasturbocharger as defined in claim 13, wherein two of the radial bearingsare present, between which the at least one axial bearing is arranged.17. The exhaust gas turbocharger as defined in claim 13, wherein eachbearing ring is enclosed on both sides by the radial bearing stators.18. The exhaust gas turbocharger as defined in claim 17, wherein eachtwo of the radial bearing stators are combined into a yoke that isU-shaped in cross section.
 19. The exhaust gas turbocharger as definedin claim 13, wherein several of the permanent magnets are arranged nextto one another in the radial direction.
 20. The exhaust gas turbochargeras defined in claim 19, wherein the permanent magnets on the radialbearing stator and the bearing ring are in contact with one another inthe radial direction.
 21. The exhaust gas turbocharger as defined inclaim 19, wherein each two of the adjacent permanent magnets in theradial direction are oppositely polarized.
 22. The exhaust gasturbocharger as defined in claim 13, wherein the at least one radialbearing stator is supported via spring and damper elements on ahousing-mounted part of the exhaust gas turbocharger.
 23. The exhaustgas turbocharger as defined in claim 22, wherein the spring elements areembodied as axially extending torsion springs.
 24. The exhaust gasturbocharger as defined in claim 23, wherein each radial bearing statoris connected to the housing-mounted part via the torsion springsdistributed over the circumference.
 25. The exhaust gas turbocharger asdefined in claim 24, wherein the torsion springs are each part of a cagethat connects the ends of the torsion springs via cage rings.
 26. Theexhaust gas turbocharger as defined in claim 25, wherein the cagesurrounds the respective radial bearing stators.
 27. The exhaust gasturbocharger as defined in claim 22, wherein the radial bearing statoror stators are braced against the housing-mounted part by way of atleast one damping element.
 28. The exhaust gas turbocharger as definedin claim 27, wherein the damping element is embodied annularly andcoaxially with respect to the shaft.
 29. The exhaust gas turbocharger asdefined in claim 27, wherein the damping elements are embodied as liquidfilms.
 30. The exhaust gas turbocharger as defined in claim 29, whereinthe liquid films contain magnetic or magnetizable particles and aremagnetically impinged upon on at least one side by a permanent magnet.31. The exhaust gas turbocharger as defined in claim 30, wherein thepermanent magnets are part of the radial bearings.
 32. An exhaust gasturbocharger comprising: a shaft; turbine wheel on the shaft; acompressor wheel on the shaft; passive magnetic radial bearings guidingthe shaft, the radial bearings comprising first permanent magnetsgenerating axial magnetic fluxes; and at least one active magnetic axialbearing guiding the shaft, the axial bearing comprising a radiallyprojecting bearing ring on the shaft made of magnetizable material, atleast one yoke made of ferromagnetic material and forming at least oneaxial bearing stator that encloses the bearing ring to establishmagnetic gaps, at least one pair of axially oppositely polarized secondpermanent magnets arranged axially next to one another in the yoke, anelectromagnetic coil arranged radially adjacently as an electromagnet,an axial sensor, and a controller for controlling the magnetic flux inthe coil and in the magnetic gaps in such a manner that the bearing ringis held in the yoke in an axially defined position.